Method for reactive sputtering deposition

ABSTRACT

The invention is a method for obtaining a reactive sputtering process with a reduced or eliminated hysteresis behaviour. This is achieved by focusing the ion current onto a small area, a reduced erosion area ( 14 ), which is in constant motion along the target ( 10 ) to avoid melting of target material. This means that the current density is very high at the reduced erosion area ( 14 ) while the average overall current density is significantly lower. The problem with arcing during reactive sputtering will be suppressed since the compound layer is effectively removed if the current density is sufficiently high. Moreover, the high current density results in a substantial increase of the fraction of ionized

FIELD OF THE INVENTION

[0001] The present invention relates to the reactive sputteringdeposition process.

BACKGROUND OF THE INVENTION

[0002] Sputtering is a preferred industrial thin film coating process.In this process, a target material is deposited over a substrate area.By bombarding the target with gas ions accelerated by a high voltage,target atoms are caused to eject, or sputter, from the surface. Targetparticles then traverse the sputtering chamber and are deposited ontothe substrate as a thin film.

[0003] For some sputtering applications, it is in several ways favorableif a large fraction of the sputtered atoms is ionized. Firstly, thesputtered ions might be attracted to the substrate by applying a bias tothe latter. This will add energy to the growing film, which isbeneficial for the film growth. Secondly, the attracted ions will have apreferential perpendicular direction when arriving onto the substratesurface which enables deposition in groves and trenches. Thirdly, if thefraction of ions is sufficiently large, it may be possible to run theprocess in a self-sustained mode. This means that ions from the targetmaterial sputter themselves without addition of an extra (inert) gas.This will of course result in a much cleaner process where no inert gasspecies is contaminating the deposited film.

[0004] The fraction of ionized sputtered species is correlated to thetarget ion current density. A higher current density implies a largerfraction of ionized species. Generally, the maximum tolerable currentdensity is limited by the efficiency of the target cooling system.

[0005] By adding a reactive gas to the sputtering process, it ispossible to reactively sputter thin films consisting of oxides,nitrides, carbides etc. The reactive sputtering process has foundwidespread applications, in for example coating of tools, decorativecoatings, window glasses, plastic webs, electronic components,data-storage components etc. Due to its highly complicated behaviour,the reactive sputtering process is associated with a number ofdifficulties. The process usually exhibits a behaviour presenting ahysteresis effect, which makes it difficult to control. Moreover, thedeposition rate of compound is usually much lower than the depositionrate of pure metal (sometimes as much as 10-20 times lower). Finally,deposition of insulating compound thin films from metal targets impliescharging and subsequent arcing at the target.

[0006] The only way to avoid hysteresis in the reactive sputteringprocess, so far, has been to increase the external pumping speed ofreactive gas. This may sometimes be realized for small systems but leadsto unrealistically high pumping speeds in large industrial systems.

[0007] In an ideal controllable process, reactive sputtering would becarried out from a clean metal target and the metal atoms would reactwith the gas when arriving at the substrate. It would then be possibleto change the compound concentration in the deposited film by thereactive gas flow. Unfortunately, it is not possible to obtain theseideal conditions because the reactive gas also reacts with the target,resulting in compound formation at the target. Experiments havepreviously been carried out in order to introduce a pressure gradient inthe processing chamber and thereby reduce compound formation at thetarget. Such a gradient is not easily obtained and the impact on theprocess behaviour has so far been quite small.

[0008] To solve the problem with charging and arcing during depositionof insulating films, it is possible to make use of specially developedpower supplies which add extra positive pulses in order to neutralizethe target.

[0009] Another way to partly overcome this problem is to increase thetotal gas pressure so that a substantial fraction of the sputtered metalfrom the high-erosion parts of the target scatters back onto thelow-erosion parts of the target, thus making this part more metallicwhich suppresses charging and subsequent arcing.

[0010] The concept of using a magnet that is moving relative to thetarget to induce a moving erosion area is well known and described inseveral patents and patent applications, see for example U.S. Pat. No.6,183,614, WO 01/23634 and WO 92/02659. In U.S. Pat. No. 6,183,614,asymmetric magnetic fields are introduced in order to achieveadvantageous high-density plasma sputtering. WO 01/23634 uses aplurality of magnets to induce a magnetic field having a predefinedarbitrary shape and thereby improve material utilization. An alternativetarget set-up is disclosed in WO 92/02659, which employs a cylindricalmagnetron for the purpose of reducing arcing.

[0011] Problems associated with prior art sputtering processes remainand there is a strong demand for an improved sputtering method.

SUMMARY OF THE INVENTION

[0012] A major object of the present invention is to provide a high-ratereactive sputtering process with a reduced or eliminated hysteresisbehaviour. This implies a more controllable and efficient reactivesputtering process, where the deposition rate of compound films issignificantly higher than in prior-art methods.

[0013] This object is achieved in accordance with the appended claims.

[0014] The invention is a method for obtaining a reactive sputteringprocess with a reduced or eliminated hysteresis behaviour. This isachieved by increasing the ion target current density in the sputteringprocess by reducing the area from which sputtering takes place. Tocompensate for the increased local heating, the reduced area from whichsputtering takes place moves along the target.

[0015] It should be noted that for certain combinations of targetmaterials and requirements on the deposition rate, the target will notmelt during operation even at static conditions, in particular if thepower is switched on and off, so that the average power is less than thepower needed to cause local heating of the target. Nevertheless, themaximum deposition rate can always be increased if the power is spreadout over a larger area. It is however crucial that the reduced area, inaccordance with the present invention, is small enough to obtain areactive sputtering process with eliminated or at least considerablyreduced hysteresis. It is also, especially in connection with industrialapplications with high requirements for efficient processes, importantthat the reduced area is moving at a speed high enough to prevent localmelting of the target. Moreover, the speed of the reduced area should below enough to make sure that the vast majority of the sputtered speciesin the reactive sputtering process are metal atoms.

[0016] The method according to the present invention has severaladvantages compared to conventional sputtering techniques. As aconsequence of the high current density, the fraction of ionizedsputtered species will increase dramatically, which is desirable for thereasons given above. Another major advantage is that the above-describedproblems associated with a sputtering process operating in reactivemode, particularly the problem with hysteresis, will be reduced oreliminated. Furthermore, the problem with arcing during reactivesputtering will be suppressed, since the insulating compound material atthe target is effectively sputter-removed when the current density issufficiently high. Finally, the increased ionization rate enablessputtering at lower pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention, together with further objects and advantagesthereof, may best be understood by making reference to the followingdescription taken together with the accompanying drawings, in which:

[0018]FIG. 1 illustrates the erosion track on a typical prior artDC-magnetron target;

[0019]FIG. 2 is an exemplary illustration of a reduced area in constantmotion according to the present invention;

[0020]FIG. 3 shows reactive gas pressure, sputtering rate and target-and substrate composition versus reactive gas flow in a reactivesputtering system using a conventional target of normal size;

[0021]FIG. 4 shows reactive gas pressure, sputtering rate and target-and substrate composition versus reactive gas flow in a reactivesputtering system with a small sized erosion area in accordance with thepresent invention;

[0022]FIG. 5 illustrates the situation at the target surface in areactive sputtering process when a reduced erosion area moves along itat a speed v, whereby the thickness of the compound layer on the targetsurface is shown in a diagram as a function of the position on thesurface;

[0023]FIG. 6 illustrates the situation at the target surface in areactive sputtering process when a reduced erosion area moves along itat a speed 3v, whereby the thickness of the compound layer on the targetsurface is shown in a diagram as a function of the position on thesurface; and

[0024]FIG. 7 is a flow chart over a method for reactive sputtering inaccordance with the present invention.

DETAILED DESCRIPTION

[0025]FIG. 1 illustrates the erosion track on a typical prior artDC-magnetron target. Sputtered material is removed from a target 10,whereby an erosion track 12 is formed at the surface of the target 10.Conventional DC-magnetron targets 10 typically have ring-shaped erosionareas 12 extending over a substantial part of the target. Other shapesof targets and erosion areas are also quite frequently used inindustrial applications. In the large area deposition industry, it iscommon to use cylindrically shaped targets. Here, the sputtering takesplace from the outer parts of the target from an erosion area parallelto the cylinder-axis. This can be achieved by inserting rod magnetsinside the cylindrically shaped target.

[0026] The present invention is based on a method to obtain a reactivesputtering process with a reduced or eliminated hysteresis behaviour byreducing the area from which sputtering takes place. Any conventionalpower supply can be used to generate the ion current to the target, forexample DC, RF or pulsed DC power supplies. FIG. 2 is an exemplaryillustration of a reduced erosion area in constant motion according tothe present invention. To avoid local heating of the target 10 thereduced erosion area 14 should be in constant motion, which in FIG. 2 isindicated by an arrow. Other embodiments of the present invention maydisclose other shapes of the reduced erosion area 14, which does notnecessarily have to be circular. However, it is crucial that the reducederosion area 14 is in constant motion to avoid local melting of thetarget 10. This ensures that the power is spread out so that the averageheating over the area of the target 10 is essentially the same as forthe large erosion area 12. Of course, embodiments with other types oftargets, such as the previously mentioned cylindrical target, may alsobe used in the present invention, as long as the erosion area is reducedsufficiently to obtain the desired process behaviour.

[0027] There are several ways of accomplishing the small erosion area 14in motion in accordance with the present invention. One approach is toplace a small moving magnet behind the target 10. Another approach is toarrange a moving shutter with a small aperture close to the surface ofthe target 10. The shutter will effectively prevent plasma dischargeexcept at the position of the aperture. For cases where magnets and/orshutters or the like are used to define the erosion zone, moving thetarget is equivalent to moving the magnets/shutters. Yet another way ofobtaining the desired small erosion area 14 in motion would be to splitup the target 10 in several parts electrically isolated from each other.By distributing power to one part at a time, it is possible to controlthe location of the plasma discharge and thereby the area from whichsputtering takes place similarly as in the previous approaches. Otherpossible approaches to obtain a small erosion area 14 in motion alongthe target 10 also lie within the scope of the present invention.

[0028] There are two crucial parameters to control in the methodaccording to the present invention; the size of the reduced erosion area14 and the speed at which the area 14 is moving along the target 10. Aswill be evident in the following text, several effects related to theseparameters have to be considered in order to obtain a hysteresis-freereactive sputtering process.

[0029]FIG. 3 shows results from simulations of a reactive sputteringsystem with a target and erosion area of normal size while FIG. 4 showssimulations of a system with a significantly smaller erosion area. Theerosion area resulting in FIG. 4 is approximately 30 times smaller thanthe one resulting in FIG. 3. Graphs of partial reactive gas pressure,sputtering rate and fraction of compound are shown. It is evident thatthe hysteresis behaviour disappears when the erosion area issignificantly reduced. In fact, it can be shown that the hysteresisbehaviour can always be eliminated by a sufficiently small erosion area.It should be noted that increasing the current density by increasing thetotal current while maintaining a large erosion area would not eliminatethe hysteresis behaviour. Accordingly, it is from this point of viewbeneficial to have as small erosion area as possible. But because ofcooling problems it is not practicable to have an arbitrarily smallerosion area. Under certain circumstances it is equivalent to make useof the moving erosion area 14 described above, instead of using theconventional target 10 with a static erosion track 12 of the same size.But for reasons given above, a moving erosion area can be made muchsmaller than a static one. This small erosion area has the advantagethat a high current density is achieved, which in turn gives ahysteresis-free process as seen in FIG. 4.

[0030] It is not possible to precisely specify the size of the erosionarea required to obtain the desired positive effects, since this sizedepends on the employed type of sputtering deposition system. To findout the size needed to obtain the positive effects, one has to reducethe erosion area and then study the hysteresis behaviour for theparticular system, in accordance with methods well-know to the manskilled in the art. If the hysteresis effect has been eliminated, ordecreased to such an extent that deposition of fully stoichiometriccompound films is achievable at a rate close to a metallic sputteringrate, it can be concluded that the erosion area has been reducedsufficiently to obtain the desired effect. The same effect may of coursealso be obtained if several erosion zones are used instead of one, aslong as the total area does not exceed the size needed to obtain thiseffect. The reduced erosion area can be moved continuously over thesurface at constant or variable speed. According to one embodiment ofthe invention, the erosion zone is caused to jump from one position toanother, e.g. by splitting up the target into electrically isolatedparts and distributing power to one part at a time.

[0031]FIG. 5 illustrates the situation at the target surface in areactive sputtering process when the erosion area moves along it at aspeed v. It is seen that a certain amount of compound has formed in thearea in front of the erosion area. An arbitrary point at the targetsurface will have this amount of compound before being bombarded by theions for a certain time. This time is equal to the speed of the erosionarea multiplied by its length. As the point enters the erosion area 14,the compound will gradually be sputtered away until a new compositionalbalance with a substantially lower amount of compound, is reached. Whenno longer bombarded by ions, the target surface will gradually be morerich in compound again as the reactive gas forms a compound with themetal of the target 10.

[0032] When sputtering is performed with a moving erosion area 14, oneimportant criterion to fulfil is that the compound rich part of thetarget 10 that enters the erosion area 14 is sputtered a sufficientlylong time to reach the new compositional balance, which is more rich inmetal. In fact, it is crucial that the new compositional balance isreached quite early during the sputtering pulse. In other words, thespeed of the erosion area 14 must be low enough to make sure that thetarget surface is sputtered a relatively short time in compound mode,denoted C in FIG. 5, but most of the time in metal mode, denoted M inFIG. 5. This criterion puts restrictions upon the maximum speed of theerosion area 14. A diagram in FIG. 5 shows the composition on the targetsurface as a function of the position on the surface-of the target 10,when the erosion area moves at the speed v.

[0033]FIG. 6 illustrates the situation at the target surface in areactive sputtering process when the erosion area is scanning it at aspeed 3v, whereby the thickness of the compound layer on the targetsurface is shown in a diagram as a function of the position on thesurface. The same denotation as in FIG. 5 is used. By comparing FIGS. 5and 6, it is seen that when the erosion area 14 moves at the speed v,most of the erosion area is in metal mode, while it is mostly compoundwhen the speed is 3v. Thus, the maximum speed of the erosion area 14should in this case be somewhere between v and 3v for a preferredreactive sputtering process.

[0034] Furthermore, the speed of the erosion area 14 has to be higherthan a certain minimum speed in order to avoid local excessiveoverheating or melting of the target 10.

[0035] If the speed of the erosion area 14 is above the minimum speedrequired to avoid melting and below the maximum speed necessary tomaintain a major part of the erosion area 14 in metal mode, the overallreactive sputtering process will behave similar to a process with atarget 10 having a static erosion track with the reduced size. Sincesimulations of a small sized static erosion track clearly show that thehysteresis effect disappears, it follows that a sufficiently smallmoving erosion area 14 also will give a hysteresis-free process providedthat the speed is within the above limits. A hysteresis-free reactivesputtering process is very desirable, since it is easy to control.Furthermore, the problem with arcing during reactive sputtering willdisappear, since the insulating compound material at the target iseffectively sputter-removed when the current density is sufficientlyhigh.

[0036]FIG. 7 is a flow chart over a method for sputtering in accordancewith the present invention. The sputtering process starts in a step S1.To obtain a hysteresis-free reactive sputtering process, the target isarranged to focus the ion current on an erosion spot in a step S2. Thearea of the erosion spot is small enough to provide a reactivesputtering process with a reduced or eliminated hysteresis behaviour, orwith a high fraction of sputtered species consisting of unreacted metalenabling deposition of fully stoichiometric compound films at a rateclose to a metallic sputtering rate. Movement of the erosion spot iscaused in a step S3. The speed with which the erosion spot is moved ishigher than the minimum speed required to avoid melting of targetmaterial and lower than a maximum speed in order to maintain a majorpart of the erosion spot in metal mode. The sputtering process is endedin a step S4.

[0037] It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope and spirit thereof, as defined by the appendedclaims.

1. A method for improving a reactive sputtering process involving a target (10), characterized by the steps of: arranging a reduced erosion area (14) on the target (10), the size of the reduced erosion area (14) being smaller than a critical size needed for a reactive sputtering process with a reduced or eliminated hysteresis behaviour or a reactive sputtering process where a fraction of sputtered species consisting of unreacted metal is so high that deposition of fully stoichiometric compound films is achievable at a rate close to a metallic sputtering rate, and causing movement of the reduced erosion area (14) along the target (10) at a constant or variable speed, the speed being selected above a minimum speed to avoid melting of target material and thereby increase a maximum tolerable current density of the target (10), and below a maximum speed to maintain a major part of the reduced erosion area (14) in metal mode.
 2. The method according to claim 1, characterized in that the step of arranging comprises arranging at least one magnet behind the target (10) and imposing a relative movement between the magnet and the target (10).
 3. The method according to claim 1, characterized in that the step of arranging comprises arranging a shutter with an aperture in front of the target (10) and imposing a relative movement between the shutter and the target (10).
 4. The method according to claim 3, characterized in that the target has a cylindrical shape and sputtering takes place at the outer parts of the target.
 5. The method according to claim 1, characterized in that the step of arranging comprises splitting up the target (10) into electrically isolated parts and distributing power to one part at a time. 